Enzymatic Catalysis of Proton Transfer and Decarboxylation Reactions.

Deprotonation of carbon and decarboxylation at enzyme active sites proceed through the same carbanion intermediates as for the uncatalyzed reactions in water. The mechanism for the enzymatic reactions can be studied at the same level of detail as for nonenzymatic reactions, using the mechanistic tools developed by physical organic chemists. Triosephosphate isomerase (TIM) catalyzed interconversion of D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate is being studied as a prototype for enzyme catalyzed proton transfer, and orotidine monophosphate decarboxylase (OMPDC) catalyzed decarboxylation of orotidine 5'-monophosphate is being studied as a prototype for enzyme-catalyzed decarboxylation. 1H NMR spectroscopy is an excellent analytical method to monitor proton transfer to and from carbon catalyzed by these enzymes in D2O. Studies of these partial enzyme-catalyzed exchange reactions provide novel insight into the stability of carbanion reaction intermediates, that is not accessible in studies of the full enzymatic reaction. The importance of flexible enzyme loops and the contribution of interactions between these loops and the substrate phosphodianion to the enzymatic rate acceleration are discussed. The similarity in the interactions of OMPDC and TIM with the phosphodianion of bound substrate is emphasized.